Heating, ventilating, and air conditioning or cooling (HVAC) systems can be used to regulate the environment within an enclosed space. Typically, an air blower is used to pull air (i.e., return air) from the enclosed space into the HVAC system through ducts and push the air back into the enclosed space through additional ducts after conditioning the air (e.g., heating, cooling or dehumidifying the air). Unless otherwise indicated, as used throughout this document, “or” does not require mutual exclusivity. Various types of HVAC systems may be used to provide conditioned air for enclosed spaces.
For example, some HVAC units are located on the rooftop of a commercial building. These so-called rooftop units, or RTUs, typically include one or more blowers and heat exchangers to heat or cool the building, and baffles to control the flow of air within the RTU. Some RTUs also include an air-side economizer that allows selectively providing fresh outside air (i.e., ventilation or ventilating air) to the RTU or to recirculate exhaust air from the building back through the RTU to be cooled or heated again.
At least one type of an economizer includes two damper assemblies driven by a common actuator. The damper blades are linked such that when the outdoor damper is open, the return air damper is closed. When a building is occupied, the outdoor damper of the economizer is typically opened a small amount (e.g., ten to twenty five percent) to allow fresh air into the building to meet ventilation requirements. When the outdoor air is colder than the return air and cooling is needed, the outdoor damper is typically opened to a hundred percent to allow the cooler outdoor air to enter the building. These two functions of an economizer are often referred to as a ventilation mode and a free cooling mode, respectively.